In October, 1957, a twenty-three inch diameter metal ball made history. Named Sputnik 1, it was the first communication satellite successfully launched into orbit of the Earth. Sputnik was the first, but by far was not the last. In the decades since then, thousands of satellites have been launched, and they have grown to become a critical and reliable aspect of our society. Today, they are used to enable rapid distribution of newspaper content, television services, global positioning information, radio stations, and countless other types of information.
One of the more prominent uses of satellites today is in the transmission of television programming and content in a digital format. Customers may purchase individual satellite dishes to subscribe to any of a number of these satellite digital television services. The present discussion relates to several technical standards that are used to enable such services. The first such standard deals specifically with satellite transmission, was established by the European Telecommunications Standards Institute (ETSI), and is known as DVB-S. The DVB-S standard focuses on satellite transmission of digital video data, and employs another digital standard, known as MPEG2, for the compression and packetization of the digital data carried by DVB-S transmissions. The DVB-S standard is actually a part of a family of standards established by ETSI, known generally as DVB, that are used for the transmission of video, audio and other data using the MPEG2 standard over a satellite. A similar DVB standard, known as DVB-T, is used for transmission over terrestrial links.
The MPEG2 standard, established by the Moving Pictures Experts Group, relates generally to the compression and packetization of digital data. Originally developed for use in transmitting digital video and audio data, the MPEG2 standard has also been used to transmit data by placing data packets (also known as datagrams) within the payload sections of one or more MPEG2 packets, which are transported as MPEG2 transport stream(s) according to the DVB-S standard. The part of the DVB standard that defines the transmission of data protocols over a DVB carrier is called Multi Protocol Encapsulation (MPE). This option can be used to transmit IP (Internet Protocol) packets thus allowing satellite-based connectivity to the Internet. As an extension to this possibility, it is also possible to use this satellite transmission to enable voice over IP (VoIP) telephony applications.
Such satellite-based telephone applications are advantageous, in that they allow telephone service and communications for remote locations in the world, or areas that lack the infrastructure for wired communications.
Using the DVB-S standard to carry voice over IP allows cheaper systems to be manufactured, since much of the DVB-S and IP-based equipment is already in use for other Internet purposes, and the economy of scale drives prices downward. There are problems, however, with such satellite telephone solutions. Namely, using the DVB-S and IP protocols requires significant overhead.
For example, in a typical voice over IP system, voice data is compressed into voice packets that are 24 bytes long. Each such voice packet carries the information for 30 milliseconds of voice data. To help reduce overhead, a technique called “packing” is often used to place two voice packets within a single MPEG2 packet bound for a single destination. Since each voice packet carries 30 milliseconds of data, carrying two voice packets in each MPEG2 packet would mean that each MPEG2 packet must be processed once every 60 milliseconds, or else a perceivable delay will result in the audio.
This “packing” of pairs of voice packets reduces some overhead, but much overhead remains. The IP protocol requires 20 bytes of overhead to contain the necessary IP routing and addressing information associated with the two 24-byte voice packets. Transmitting these voice packets requires another 12 bytes per transmission for the Real Time Transport Protocol (RTP), as well as another 8 bytes for the User Datagram Protocol (UDP). Accordingly, to transmit 48 bytes of actual data (the two 24-byte packets), there is a corresponding additional 40 bytes of overhead.
Typically, the MPE format is used to encapsulate IP datagrams for transmission using the DVB format. This MPE encapsulation adds a-12 byte header (including, among other, the MAC address and length) and a 4-byte Cyclic Redundancy Check (CRC) for error detection. Therefore, 16 bytes are added to each IP datagram to be sent over DVB using MPE encapsulation. The total overhead is determined by the length of the IP datagram, if a single datagram is encapsulated in an MPEG2 packet then we get 16 bytes for the MPE encapsulation header. Since each MPEG2 packet has a fixed length of 188 bytes, then if small datagrams are to be sent, then multiple datagrams can be packed in a single MPEG2 packet, this is called packing. When packing is used the MPE overheads are 16 bytes per packet. Very large datagrams are split into multiple MPEG2 packets, but the MPE overhead is still 16 bytes. Therefore the MPE encapsulation scheme is efficient for large IP datagrams. However for telephony applications, where the datagrams are very small (48 bytes), the MPE is not very efficient.
Totaling these figures, to transmit 48 bytes of data in existing satellite-based voice-over-IP systems, an additional 56 bytes of overhead is required. This overhead consumes the transmission capacity of a satellite, reducing the amount of actual voice data that it can transmit at any one time. There is a need to reduce this overhead, and maximize the effective use of a satellite's transmission capability to provide telephony services.